Prior art machines for making filter bags from a web of filter paper comprise a plurality of stations, including a station for feeding a web of filter paper, a folding station and a first station for longitudinally sealing the folded web to form a continuous tubular wrapping of filter paper and preferably for transversally sealing the continuous tubular wrapping to form a succession of filter bags of substantially flattened shape.
The longitudinal and transversal sealing station may comprise a specially shaped upper roller with tracks for longitudinally and transversally sealing the continuous tubular wrapping of filter paper. The seals may be made by heat sealing means or, in the solutions of the latest generation, by sonotrodes, positioned in contact with the product to be sealed.
The sonotrode is the transducer element that converts the power supply into vibrations that are discharged onto the product to be sealed.
Prior art sealing devices, however, present some disadvantages linked to the fact that the edge of the product, after being sealed, is irregular and therefore requires trimming, that is to say, an additional cutting operation to obtain a straight, regular edge.
This constitutes a particularly serious drawback at high production speeds when it is even more difficult to guarantee a regular seal, and especially in cases where the quality of the sealed finish has a significant commercial value.
A first aim of the present invention is to provide an ultrasound sealing device that overcomes the above mentioned disadvantages.
A second aim of the invention is to reduce the number of separate steps required to obtain a finished product, for example, in the production of bags from a web of material, whether tubular or not.